Future sand dynamics of the Mekong Delta

Abstract

As sand is a key building element of deltas, human interference with sand transport alters the deltas morphology. In the Mekong Delta, upstream sediment trapping by dams and local sand mining practises have already proven to rapidly change the deltas morphology in the form of river bed deepening and coastal retreat, resulting in land loss and increased salinisation in the delta. Further processes that contribute to these problems in the delta are delta-wide subsidence due to groundwater extraction and global sea level rise (SLR). This study aims to provide an overview on the projected changes in sand transport mechanisms due to SLR, subsidence, river bed lowering and discharge alteration by dams. For this, a delta-wide DELFT3D-FM hydro- and morphological model is used to illustrate the current complex sand dynamics inside the delta. This study shows the influence of human impact on the deltas internal sand dynamics by separately testing the systems response to SLR sea level, subsidence, river bed lowering and an altered discharge. In the end, projected changes on sand dynamics for 2040 were simulated. The results confirm the different distinct hydrodynamic regimes present in the delta. In the fluvial dominated tract seaward sand transport is highly variable and rates are 1-2 orders of magnitude larger than in the estuaries downstream. Here, a reduced wet season discharge results in lower net annual sand transport volumes. Deepening the river bed lowers water levels relative to the banks, effectively capturing the peak flow, reducing the deltas buffering capacity. This ultimately increases the seasonality of the sand transport in this region. In the fluvial-tidal regime, seaward sand export increases by the amplified tidal velocity amplitude as a result of SLR, subsidence and river bed deepening. In the tide-dominated regime, seaward sand export rates decrease as increased water depth in the wide estuaries results in a large increase in the estuaries cross-sectional area, reducing the tidal flow velocity amplitude. Sand influx from the Mekong is expected to decrease as wet season discharge is reduced. Furthermore, increased erosion rates can be expected near My Thuan, contributing to the already present river-bank instability. Lastly, the low transport rates in the smaller estuaries show that sand replenishment is slow, therefore exploitation in these regions will have long-term consequences for the sand availability here. The results of this study can be used to identify locations where morphological changes can be expected in other deltas worldwide.